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Rare species are crucial components of the highly diverse soil microbial pool and over-proportionally contribute to the soil functions. However, much remains unknown about their assembling rules. The biogeographic patterns and species aggregations of the rare bacterial biosphere were assessed using 140 soil samples from a gradient of 2000 km across the main tea-producing areas in China. About 96% OTUs with ~40% sequences were classified as rare taxa. The rare bacterial communities were significantly affected by geographical regions and showed distance-decay effects, indicating that the rare bacteria are not cosmopolitan, they displayed a pattern of limited dispersal and were restricted to certain sites. Variation partitioning analysis (VPA) revealed that environmental variation and spatial factors explained 12.5% and 6.4%, respectively, of the variance in rare bacterial community. The Mantel and partial Mantel tests also showed that the environmental factors had stronger (~3 times) impacts than spatial factors. The null model showed that deterministic processes contributed more than stochastic processes in rare bacterial assembly (75% vs. 25%). There is likely an enrichment in ecological functions within the rare biosphere, considering this high contribution of deterministic processes in the assembly. In addition, the assembly of rare taxa was found to be mainly driven by soil pH. Overall, this study revealed that rare bacteria were not cosmopolitan, and their assembly was more driven by deterministic processes. These findings provided a new comprehensive understanding of rare bacterial biogeographic patterns and assembly rules. [ABSTRACT FROM AUTHOR]

Microbes are key components of the soil environment, playing an important role in maintaining soil health, sustainability, and productivity. The composition and structure of soil bacterial communities were examined in winter wheat-rice (WR) and winter wheat-maize (WM) cropping systems derived from five locations in the Low-Middle Yangtze River plain and the Huang-Huai-Hai plain by pyrosequencing of the 16S ribosomal RNA gene amplicons. A total of 102,367 high quality sequences were used for multivariate statistical analysis and to test for correlation between community structure and environmental variables such as crop rotations, soil properties, and locations. The most abundant phyla across all soil samples were Proteobacteria, Acidobacteria, and Bacteroidetes. Similar patterns of bacterial diversity and community structure were observed within the same cropping systems, and a higher relative abundance of anaerobic bacteria was found in WR compared to WM cropping systems. Variance partitioning analysis revealed complex relationships between bacterial community and environmental variables. The effect of crop rotations was low but significant, and interactions among soil properties, locations, and crop rotations accounted for most of the explained variation in the structure of bacterial communities. Soil properties such as pH, available P, and available K showed higher correlations (positive or negative) with the majority of the abundant taxa. Bacterial diversity (the Shannon index) and richness (Chao1 and ACE) were higher under WR than WM cropping systems.